1. Field of the Invention
The present invention relates generally to telephones with hands-free operation and more particularly to an improved hands-free circuit of a radio or landline telephone which may be utilized in noisy environments.
2. Description of the Related Art
In a radio telephone system, a user communicates with another user by means of a telephone handset. In operation one hand of the user has to be used to hold the handset. Therefore only the other hand of the user is free to drive additional vehicle. In order to provide a freedom for the user, hands-free circuits have been developed for the radio telephone.
A basic mobile radio telephone system is shown in FIG. 1. In the conventional radio telephone systems, a mobile party in a vehicle 1 communicates with a radio base station 5 using a mobile radio 3. The base station 5 communicates with a land party (another user) using a radio system control terminal 7.
In the conventional radio telephone, an acoustic coupling from a speaker 15 to a microphone 13 may occur. Further the audio signal from the microphone 13 may be reflected in the mobile radio 3. Therefore a loop from the speaker 15 to the microphone 13, the mobile radio 3 and the speaker 15 is generated. If the amplification gain of the loop is great, a howling occurs in the speaker 15.
Referring to FIG. 2, there is illustrated a block diagram of a conventional hands-free control circuit which has been used for avoiding the howling. The control circuit of FIG. 2 is provided in the mobile radio 3.
The control circuit interfaces a microphone 13 and a speaker 15 to a mobile radio transmitter 51 and a mobile radio receiver 53, respectively. A path from the microphone 13 to a mobile radio transmitter 51 is defined as a microphone audio path. A path from a mobile radio receiver 53 to the speaker 15 is defined as a speaker audio path. In the microphone audio path, an amplifier 61 and an attenuator 55 are provided. In the speaker audio path, an attenuator 57 and an amplifier 63 are provided.
The audio signal from the attenuator 55 is applied to the mobile radio transmitter 51. The mobile radio transmitter 51 generates a radio signal from the audio signal. The radio signal is transmitted to the base station via duplexer 65 and an antenna 66. A radio signal transmitted from the base station 5 is received through by the antenna 66, the duplexer 65 and the mobile radio receiver 53. The mobile radio receiver 53 demodulates an audio signal from the radio signal. The audio signal from the mobile radio receiver 53 is applied to the attenuator 57 and a recieve signal level detector 67. The recieve signal level detector 67 detects a level of the audio signal from the mobile radio receiver 53.
The audio signal generated by the amplifier 61 in the microphone audio path is applied to the attenuator 55 and a transmit signal level detector 69. The transmit signal level detector 69 detects a level of the audio signal from the amplifier 61.
A comparator 59 provides a binary one state if the output level of the transmit audio signal level detector 69 is higher than the output level of the receive audio signal level detector 67. This state is defined as a transmit-mode. The binary one state from the comparator 59 is applied to the attenuator 57 and an inverting gate 71. Therefore the attenuator 57 is activated for attenuating the audio signal from the mobile radio receiver 53. The attenuator 55 is not activated due to the inverting gate 71.
The comparator 59 provide a binary zero state if the output level of the receive audio signal level detector 67 is higher than the output level of the transmitt audio signal level detector 69. This state is defined as a receive-mode. The binary zero state from the comparator 59 is applied to the attenuator 57 and the inverting gate 71. Therefore the attenuator 55 is activated for attenuating the audio signal from the amplifier 61. The attenuator 57 is not activated due to the inverting gate 71.
Thus since the amplification gain of the loop from the speaker 15 to the microphone 13, the mobile radio transmitter 51, the mobile radio receiver 53 and the speaker 15 is decreased by the attenuators 55 or 57, the howling is avoided.
Further, bias voltages are applied to the transmit audio signal level detector 69 and to the receive audio signal level detector 67 by an offset voltage circuitry 73, such that the bias voltage to the transmit audio signal level detector 69 is higher than the bias voltage to the receive audio signal detector 67. When there is no input to the microphone 3 and no audio signal from the mobile radio receiver 53, therefore, the comparator 59 provides a binary one state.
In the conventional circuit, however, the comparator 59 provides the output by comparing the levels detected by the transmit audio signal level detector 69 and the receive audio signal level detector 67. Therefore, when the ambient noise occurs in a vehicle, the output level of the transmit audio signal level detector 69 is higher than the output level of the receive audio signal level detector 67. That is, although a receive audio signal from the mobile radio receiver 53 is greater than a voice signal from the microphone 13, the ouput level of the transmit signal level detector 69 is made greater than the ouput level of the detector 67 due to a noise signal from the amplifier 61 to the detector 69. Therefore when the noise starts occuring, the ouput of the comparator 59 changes to the binary one state from the binary zero state. As a result, the comparator 59 activates the attenuator 57 to attenuate the receive audio signal from the mobile radio receiver 53.
That is, although another user speaks, the receive audio signal is caused to be small. Therefore the mobile party has difficulty in listening to another user's speech via the speaker 15.
Further, although another user's speech starts and the level of the ambient noise around the mobile party is much greater than the level of the receive audio signal, there is no change from the transmit mode to the receive mode. Therefore the mobile party has difficulty in listening to another user's speech.